I love sci fi. Really. While countless people whine about the lack of accuracy in sci fi generally, I’ve come to realise that suspending my disbelief is usually more fun★. Anyway, if we’re honest, deconstructing science in movies is like shooting fish in a barrel. With a bazooka. I always find it more fun to ponder the ‘what if’ of sci fi films.
And this is exactly what I found myself doing on Saturday evening. Some evenings, nothing’s nicer than relaxing with a movie, and the one in question is Pitch Black. Apparently, most people hate this film. Personally, I rather like it. Oh, and normally at this point I’d warn you about spoilers… But seriously, the film’s like, over a decade old. If you haven’t seen it by now, chances are you don’t care much about spoilers anyway.
So one of the reasons I like this film is that it’s an interesting little scenario, astronomically speaking. The background is that a space ship crash lands on a desert planet, and before very long, the survivors realise that the world they’re on (which appears to be a small deserty moon orbiting a giant planet) has three suns and hence very little night time. As you can guess by the title of the film, darkness does happen evenually, and needless to say bad things happen at night✦. Any sci fi where the multiple suns in the sky are more than just decoration gets bonus kudos from me. So can a planet like the one in this film actually exist?
Some people will, at this point, start desperately wanting me to say no. In truth, a flat no isn’t quite accurate. Could a planet be between two or more stars so that it has perpetual daylight? No. Could a planet exist in a triple star system containing a red and yellow close binary and a more distant blue star? Apparently yes, yes it could. I’m going to completely ignore the question of how an oddball star system like this might form (a can of worms which I’m not sure I’m qualified to open) and just assume that somehow it can. So for a planet to be habitable, the binary pair would be the safest component of the system to orbit. Blue stars are hotter and more dangerous, so it would be safest to keep that one a bit further away.
So for the sake of argument, let’s say that the binary are a yellow dwarf and a red dwarf. With the extra sunlight from all three stars, let’s put the planet (complete with moon) about 2 astronomical units away from the binary. I’m not sure exactly where the habitable zone would be around a pair like this, but 2 AU works as a guess for now.
So what about that bright blue star? Well, again it’s a guess (no one really bothers to calculate habitability-related things for blue stars on account of how inhospitable they are) but let’s make our planet at least 10 AU away from it. That would give the orbit between binary and blue a semimajor axis of about 12 AU (the distance from the Sun to somewhere past Saturn). Incidentally, I’m not bothering with any actual celestial mechanics for the binary pair. These things can be ridiculously close, so when you’re just fooling around with maths like this, it’s easiest to just assume they’re close together, and treat them as one object.
Ok, so now we have that all clear, let’s start throwing in a few numbers. Smallest first, a red dwarf is an M-type star, which can have a mass up to 0.45 M☉ (that’s 0.45 solar masses). Let’s give it 0.2 M☉, fairly large for a red dwarf. Let’s make the yellow star an early K-type (orange dwarf) with 0.8 M☉. Finally, that blue. Can’t make it too massive or it would fry everything on the planet. And then probably evaporate the planet. Let’s say 2.2 M☉, making it a late B-type star. Blue, but possibly not too deadly. Incidentally, B-type stars put out enough ultraviolet to give a particularly lethal sunburn, but let’s again assume that there’s something in our planet’s atmosphere which makes this less of an immediate-death type issue. So if we crunch a few numbers for the star’s orbits…
As you may expect, a star system like this doesn’t orbit either star individually. Instead, the B star and the binary pair orbit each other. The barycentre (centre of mass) for the system lies inbetween the two, rather closer to the B star. This is pretty much what I’d expect.
Ok, but what about that planet? Could it manage an orbit around the binary pair? Typically, the greatest concern is to do with Hill spheres. Any object has a sphere of gravitational influence, called a Hill sphere. The exact size of this varies, depending on how close any other massive objects are. This is important for our planet. Any objects within the Hill sphere of the close pair will remain in orbit around the close pair. Any objects outside it will be subject to gravitational perturbations by the B star. And by perturbations, I mean most likely being flung into interstellar space (less dramatically, this is how mass transfer occurs from one star to another). Needless to say, that’s not the outcome we’re hoping for. So again, if we crunch a few numbers…
The Hill sphere radius of the close pair works out at 6.4 AU. Which is actually a lot larger than I’d have expected. Though between them, they have quite a lot of mass even compared to that big B star, so I guess it isn’t that surprising.
In any case, satellite orbits are expected to be stable at distances of between 1/3 to 1/2 the Hill sphere radius. That actually means that our planet, at a distance of around 2 AU should, in fact, be in a fairly stable orbit. Admittely, I’ve used approximations left right and centre in calculating this. But that’s what back-of-the-envelope☀ calculations are all about.
So to answer the original question, yes a planet most likely could exist with a stable orbit in a system like this. Yes that planet would have three Suns. Would that planet be able to be positioned perfectly between those suns and have perpetual daylight? No. Not without being swiftly flung into icy oblivion, anyway.
★ Usually. Though I’ve seen a lot of films which are so laughably bad that you just can’t help yourself!
✦ As we should all know by now, they mostly come at night. Mostly.
☀ I mean that literally. All of my scrawlings were actually drawn on the back of an envelope before being digitalified. The envelope’s even still here on my desk!